Accurate measurement of movement is needed in various high technology applications. The most accurate measurement techniques are based on the use of laser interferometers.
In a Fourier Transform Infrared (FTIR) spectrometer the position of a moving mirror of an IR interferometer is determined by using a He—Ne laser to measure an optical path difference in the interferometer. The position of the moving mirror is measured only at discrete sampling points which are the zero crossing points of the laser interference signal with an interval of λ/2, where λ is the wavelength of the laser light. The movement of the moving mirror can be followed up to even ten meters by a fringe counting system.
When measuring small movements in the range up to a few micrometers the position of a moving element has to be measured continuously. An approach disclosed in U.S. Pat. No. 7,521,668 is based on the use of a Michelson interferometer for measuring the displacement of a sensor, which sensor is arranged to be moveable in response to sound waves. The light beam coming from the light source is focused on the surface of the sensor and the reference mirror of the Michelson interferometer. A phase difference between different parts of the light beam is provided e.g. by tilting the reference mirror whereby a spatial interference pattern is produced on the detectors. The movement of the sensor can then be calculated from changes in the interference pattern.
Even though the tilted Michelson interferometer in U.S. Pat. No. 7,521,668 works well in detecting small movements, it has some disadvantages. The Michelson interferometer is quite sensitive to temperature variations, mainly because the sensor and the reference mirror are situated apart from each other in different interferometer arms, and are thus susceptible to temperature changes between the arms.
The arrangement disclosed in U.S. Pat. No. 7,521,668 is also sensitive to the tilting of the sensor and the reference mirror because in practice the focus of the light beam cannot be arranged exactly on the surface of the sensor and the reference mirror. It is also a disadvantage that the reference mirror must be precisely adjusted in order to obtain accurate measurements of the movement of the sensor. A further disadvantage is that the optical components in the Michelson interferometer must be of a high quality and are thus expensive to manufacture, especially the amplitude splitting beamsplitter.